Variable gain amplifier

ABSTRACT

A variable gain amplifier device comprises a variable gain amplifier circuit which amplifies a difference between an input signal and a feedback signal to output an output signal, a feedback circuit which supplies the feedback signal to the variable gain amplifier circuit, and a controller which controls the variable gain amplifier circuit and the feedback circuit to decrease a cutoff frequency of the feedback circuit according to increase of a gain of the variable gain amplifier circuit or vice versa.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims priorityfrom, application Ser. No. 10/210,057, filed Aug. 2, 2002 now U.S. Pat.No. 6,690,232. This application is based upon and claims the benefit ofpriority from the prior Japanese Patent Application No. 2001-29536,filed Sep. 27, 2001, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable gain amplifier used forcable broadcasting, radio communications, a magnetic recorder and so on,particularly to a variable gain amplifier provided with a dc offsetcanceling facility.

2. Description of the Related Art

Generally, it cannot be avoided in many amplifiers that the dc offsetthat is error components of a bias voltage and a bias current occurs inan output stage. So far various measures have been taken in order toremove this dc offset. There is, for example, an offset canceling systemfor removing the offset using a feedback amplifier circuit. In thiscase, the low frequency domain of the frequency band of a signalamplified by a predetermined gain (referring to as a lower limitfrequency) fluctuates according to a change of the gain of the mainamplifier. The frequency band of the amplified signal is narrower asthis predetermined gain increases. Therefore, the low frequency domainof the signal that should be amplified at a high gain is not amplified,resulting in deteriorating quality of the signal.

It is an object of the present invention to provide a variable gainamplifier that can suppress fluctuation of a lower limit frequencyaccording to change of the gain of a main amplifier circuit, and realizea good offset canceling.

BRIEF SUMMARY OF THE INVENTION

According to the first aspect of the present invention, there isprovided a variable gain amplifier device comprising: a variable gainamplifier circuit supplied with an input signal and a feedback signal toamplify a difference between the input signal and the feedback signaland output an output signal; a feedback circuit which supplies thefeedback signal to the variable gain amplifier circuit; and a controllerwhich controls the variable gain amplifier circuit and the feedbackcircuit to decrease a cutoff frequency of the feedback circuit with anincrease of the gain of the variable gain amplifier circuit or viceversa.

According to the second aspect of the present invention, there is avariable gain amplifier device comprising: a variable gain amplifiercircuit supplied with an input signal and a feedback signal to amplify adifference between an input signal and a feedback signal and output anoutput signal; a feedback circuit which supplies the feedback signal tothe variable gain amplifier circuit; and a controller which controls thegain of the variable gain amplifier circuit and a cutoff frequency ofthe feedback circuit to make a lower limit frequency of the outputsignal substantially constant regardless of variation of a gain of thevariable gain amplifier circuit.

According to the third aspect of the present invention, there isprovided a variable gain amplifier device comprising: a variable gainamplifier circuit supplied with an input signal and a feedback signal toamplify a difference between the input signal and the feedback signaland output an output signal, a gain of the variable gain amplifiercircuit being varied according to a level of the input signal; and afeedback circuit which supplies the feedback signal to the variable gainamplifier circuit, a cutoff frequency of the feedback circuit beingvaried according to variation of the gain of the variable gain amplifiercircuit to make a lower limit frequency of the output signalsubstantially constant.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram of a variable gain amplifier related to anembodiment of the present invention;

FIG. 2 is a graph to show a gain-frequency characteristic of thevariable gain amplifier shown in FIG. 1;

FIG. 3 is a circuit diagram of a feedback amplifier circuit of the firstexample;

FIG. 4 is a circuit diagram of another feedback amplifier circuit;

FIG. 5 is a circuit diagram of another feedback amplifier circuit;

FIG. 6 shows a circuit diagram of a variable capacitor;

FIGS. 7A and 7B show circuit diagrams of different variable resistors;

FIG. 8 shows a circuit diagram of a voltage-to-current converter;

FIG. 9 shows a circuit diagram of another voltage-to-current converter;and

FIG. 10 shows a block diagram of a radio receiver using the variablegain amplifier.

DETAILED DESCRIPTION OF THE INVENTION

There will now be described an embodiment of the present invention inconjunction with the drawings.

As shown in FIG. 1, a variable gain amplifier comprises a main amplifiercircuit 11 of a variable gain A, a feedback amplifier circuit 12 whoseinput is connected to the output terminal of the main amplifier circuit12, and a gain control circuit 13 connected to the control inputterminals of the main amplifier circuit 11 and feedback amplifiercircuit 12. The feedback amplifier circuit 12 includes a sub-amplifiercircuit 15 of a constant gain F and a low pass filter circuit 16 of avariable pass band that is connected to the output terminal of thesub-amplifier circuit 15. The gain control circuit 13 controls the gainof the main amplifier circuit 11 and the pass band of the low passfilter circuit 16.

An input signal supplied to an input terminal 10 is input to thenon-inverting input terminal of the main amplifier circuit 11. Theoutput signal of the main amplifier circuit 11 is input to an outputterminal 14 and the feedback amplifier circuit 12. The output signal ofthe feedback amplifier circuit 12 is input to the inverting inputterminal of the main amplifier circuit 11 as a feedback signal. Thesignal input to the feedback amplifier circuit 12 is amplified by thesub-amplifier circuit 15. Only a DC component is extracted from theamplified input signal by the low pass filter circuit 16. The DCcomponent output from the feedback amplifier circuit 12 is input to theinverting input terminal of the main amplifier circuit 11. As a result,the DC component negates the dc offset component of the input signalinput to the noninverting input terminal. In this way, the dc offsetcomponent is canceled.

The gain of the main amplifier circuit 11 is controlled by the controlsignal Vca generated from the gain control circuit 13 according to asignal Vcont corresponding to a level of the output signal, and at thesame time the lower cutoff frequency of the low pass filter 16 iscontrolled by the control signal Vcf from the gain control circuit 13,too. In addition, the switching of the gain may refer to either outputor input of the main amplifier circuit 11.

When the gain control circuit 13 supplies the control signal Vca to themain amplifier 11 to increase the gain of the main amplifier circuit 11,it supplies the control signal Vcf to the low pass filter 16 to lowerthe cutoff frequency of the low pass filter 16 simultaneously. Theinput-output characteristics of this variable gain amplifier beexpressed by the following equation (1):T(s)=(A1+CRs)/(FA+1+CRs)   (1)

where A represents the gain of the main amplifier circuit 11, Fexpresses the gain of the sub-amplifier circuit 15 (the gain of bandpass of the low pass filter is 1 time), and CR expresses a time constantcorresponding to the cutoff frequency (fo=1/(2πCR)) of the low passfilter circuit 16. Assume that the main amplifier circuit 11 and lowpass filter 16 are controlled so that A/CR becomes constant since F isconstant. A big difference in transfer characteristic between the priorart variable gain amplifier and the variable gain amplifier of thepresent embodiment is that the time constant CR varies with the gain Asimultaneously. The graph expressing this condition as the frequencycharacteristic is shown in FIG. 2. In other words, 1/(2π(CR)₁) is thecutoff frequency fo indicating a DC gain T at the time when the gain Aof the main amplifier circuit 11 is A1. Similarly, fo=1/(2π(CR)₂) whenthe gain is A2, and fo=(2π(CR)₃) when the gain A is A3.

The lower limit frequency capable of maintaining the gain A (strictlythe frequency that the gain becomes A/√2 time) is expressed byAF/(2πCR). In other words, in the case of the present embodiment, it isfound that even if the gain A changes to A1, A2, or A3, the lower limitfrequency AF/(2nCR) does not vary. As alternated, the frequencyintersecting the DC gain T(0) (≅=1/F) is shifted according to the gain Ain order to vary CR according to gain A. However, this do not influencethe DC gain (dc offset attenuation) T(0).

There will now be described the feedback amplifier circuit 12 having alow pass filter function capable of changing the cutoff frequency.

The First Embodiment

FIG. 3 shows a circuit diagram of a feedback amplifier circuit 12according to the first embodiment. The feedback amplifier circuit 12comprises an operational amplifier 32, a variable capacitor 33 and afirst resistor (resistance R) 31-1 which are connected in parallelbetween the output terminal and inverting input terminal of theoperational amplifier 32, and a second resistor 31-2 connected betweenthe input terminal and the inverting input terminal of the operationalamplifier 32. Further, the output signal of the main amplifier circuit11 of FIG. 1 is input to the inverting input terminal of the operationalamplifier 32 via the second resistor (resistance R2) 31-2. A referencevoltage Vref is applied to the non-inverting input terminal of theoperational amplifier 32. This reference voltage Vref is a DC voltageused for an operation of the operational amplifier 32, and does notinfluence an operation of the variable gain amplifier of the presentinvention directly.

The low pass filter 16 comprises the operational amplifier 32, thevariable capacitor 33 and the second resistor 31-2. The second amplifiercircuit 15 comprises the first and second resistors 31-1 and 31-2 andthe operational amplifier 32.

The cutoff frequency of the filter 16 is lowered by increasing thecapacity of the variable capacitor 33 according to increase of the gainA of the main amplifier circuit 11. In addition, the gain F of thesub-amplifier circuit 15 is determined by R1/R2.

The variable capacitor 33 uses a capacitor unit wherein capacitors areswitched as shown in FIG. 6. The first terminals of a plurality ofcapacitors, for example, four capacitors 34 are connected to each other,and the second terminals of the capacitors are connected to the contactsof the switch 35 respectively. The switch 35 is switched by the controlsignal Vcf supplied from the gain division circuit 13. The switch 35 maybe constructed by CMOS transistors.

The Second Embodiment

FIG. 4 shows a circuit diagram of the feedback amplifier circuit 12according to the second embodiment. This feedback amplifier circuit 12is fundamentally the same as that shown in FIG. 3. This feedbackamplifier circuit 12 controls a resistor instead of controlling acapacitor. In other words, the cutoff frequency of the filter 16 islowered by increasing the resistance of the resistor 41-2 according toincrease of the gain of the main amplifier circuit. Also, the cutofffrequency of the filter 16 is increased by decreasing the resistance ofthe resistor 41-2 according to decrease of the gain of the mainamplifier circuit. In order to make the gain F of the feedback amplifiercircuit 12 constant, two variable resistors 41-1 and 41-2 are adjustedso as to keep the resistance ratio between the variable resistors 41-1and 41-2 at a constant value.

FIGS. 7A and 7B each show a concrete configuration of the variableresistor 41. FIG. 7A shows the variable resistor 41 wherein a pluralityof, for example, four resistors are connected in parallel and switchedby a switch 45. FIG. 7B shows the variable resistor 41 wherein aplurality of, for example, four resistors are connected in series andshort-circuited by switches 45-1, 45-2 and 45-3. The switch may beconstructed by CMOS transistors. The on-resistance of a CMOS transistoror a pseudo resistor circuit constructed by a voltage-to-currentconverter may be used instead of the variable resistor 41.

The Third Embodiment

FIG. 5 shows a circuit diagram of the feedback amplifier circuit 12related to the third embodiment. The present embodiment differs from thefirst or the second embodiment, and the feedback amplifier circuit 12comprises voltage-to-current converters 51 and 52, the mutualconductance of each of which is variable, and a capacitor 53 withoutusing the operational amplifier and resistor. The firstvoltage-to-current converter 51 converts an input signal voltage to acurrent proportional to the signal voltage. A capacitor 53 is connectedbetween the inverting and noninverting output terminals of the firstvoltage-to-current converter 51. The capacitor 53 short-circuits betweenthe input terminals of the second voltage-to-current converter 52 andbetween the output terminals thereof. This second voltage-to-currentconverter 52 acts equivallently to a resistor.

The second voltage-to-current converter 52 and capacitor 53 construct anext stage low pass filter circuit 16 having a signal gain as shown inFIG. 1. When the mutual conductances of the voltage-to-currentconverters 51 and 52 are represented by Gm1 and Gm2 respectively, thesignal gain F of the sub-amplifier circuit shown in FIG. 1 becomesGm1/Gm2. The cutoff frequency is expressed by Gm2/2πC. C expresses thecapacitance of the capacitor 53. When the gain A of the main amplifiercircuit 11 is increased by the gain control signal Vcf from the gaincontrol circuit 13, the voltage-to-current converters 51 and 52 arecontrolled by the gain control signal Vcf so that the conductances Gm1and Gm2 are decreased at the same rate simultaneously. The reason whythe conductances Gm1 and Gm2 are decreased at the same ratiosimultaneously is to keep the gain F at a constant value. Further, whenGm2 is decreased, the equivalent resistance R(=1/Gm2) increases.Therefore, the cutoff frequency (fo (=1/(2πCR)) lowers as the signalgain F of the feedback amplifier circuit 12 keeps a constant value. Onthe contrary, the feedback amplifier circuit 12 is controlled by thegain control signal Vcf so that the conductances Gm1 and Gm2 increasewhen the signal gain A of the main amplifier circuit 11 decreases.

FIGS. 8 and 9 show concrete circuits of the voltage-to-currentconverters 51 and 52 using the feedback amplifier circuit 12 shown inFIG. 5. FIG. 8 shows the voltage-to-current converter fabricated bybipolar transistors, and FIG. 9 shows the voltage-to-current converterfabricated by MOS transistors. In the voltage-to-current converter ofFIG. 8, the collectors of bipolar transistors Q1 and Q2 are connected tocurrent sources, and a resistor R11 is connected between the collectors.The emitters of the transistors Q1 and Q2 are grounded through a currentsource. The bases of the transistors Q1 and Q2 are connected to theoutput terminals of differential amplifier A1 and A2. The noninvertinginput terminals of the differential amplifiers A1 and A2 are connectedto the collectors of the transistors Q1 and Q2, respectively. Theinverting input terminal of the differential amplifier A1 is connectedto one of input terminals 55. The inverting input terminal of thedifferential amplifier A2 is connected to the base of the transistor Q2.The collectors of bipolar transistors Q3 and Q4 are connected tovariable current sources, and to output terminals 56 respectively. Theemitters of the transistors Q3 and Q4 are grounded through a variablecurrent source. The base of the transistor Q3 is connected to the baseof the transistor Q1. The base of the transistor Q4 is connected to theother of the input terminals 55. The current proportional to the signalvoltage input to the input terminal 55 is output from the outputterminal 56. The mutual conductance of this circuit is expressed by thefollowing equation (2).Gm1, Gm2=I2/(I1*R1)   (2)In other words, the conductances Gm1 and Gm2 can be controlled bychanging a ratio between currents I1 and I2. However, since the inputoperative range is determined by I1*R1, the input operative range variesif the current I1 is changed. Therefore, it is desirable to vary onlythe current I2.

In the voltage-to-current converter of FIG. 9, the drains of MOStransistors M1 and M2 are connected to current sources and to outputterminals Iout⁺ and Iout⁻ respectively. The sources of the transistorsM1 and M2 are connected to a voltage source Vss via drain-source pathsof MOS transistors M3 and M4, respectively. The gates of the transistorsM1 and M2 are connected to the output terminals of differentialamplifiers A1 and A2 respectively. The gates of transistors M3 and M4are connected to input terminals Vin⁺ and Vin⁻, respectively. Thenoninverting input terminals of the differential amplifiers A1 and A2are connected to Vcf terminals, respectively. The inverting inputterminals of the differential amplifiers A1 and A2 are connected to thesources of the transistors M1 and M2 respectively. An input voltage isapplied between the input terminals Vin⁺ and Vin⁻, and an output currentis extracted from output terminals Iout⁺ and Iout⁻.

The mutual conductances Gm1 and Gm2 of the voltage-to-current convertercan be changed by controlling the mutual conductances of transistors M3and M4. Assuming that the operating point is determined so that thetransistors M3 and M4 operate in a linear domain, the mutualconductances of the transistor M3 and M4 are proportional to thedrain-source voltages of the transistors M3 and M4. Therefore, themutual conductance is controlled by controlling the drain voltages ofthe transistors M3 and M4.

By a feedback configuration of the operational amplifiers A1 and A2 andtransistors M1 and M2, the feedback amplifier 12 operates so that thesource voltages of the transistors M1 and M2 and voltages applied to thenon-inverting input terminals of the operational amplifiers A1 and A2become equal. The drain voltages of the transistors M3 and M4 arecontrolled by the voltages applied to the non-inverting input terminalsof the operational amplifiers A1 and A2. Therefore, it is possible tochange the mutual conductances of the voltage-to-current converters 51and 52 by applying the control signal Vcf from the gain control circuit13 to the non-inverting input terminals of the operational amplifiers A1and A2.

An embodiment applied the present invention to a direct conversion radioreceiver will be described hereinafter.

According to the FIG. 10, the output terminal of a low noise amplifier101 to which a radio signal is input is connected to one input terminalof a multiplier 102. A local signal LO is input to the other inputterminal of the multiplier 102. The output terminal of the multiplier102 is connected to a variable gain amplifier 104 through a low passfilter 103. This variable gain amplifier 104 corresponds to the variablegain amplifier shown in FIG. 1. In other words, the variable gainamplifier 104 includes an amplifier 106 and a low pass filter 107connected to the output terminal of an adder 105 to which a signal fromthe filter 103 is input, an amplifier 106 connected to the outputterminal of the adder, a feedback circuit including a low pass filter107 and feeding back the output signal of the amplifier 106 to the adder105, and a gain controller 108.

According to the above radio receiver, a radio frequency signal RF isamplified by the amplifier 101, and multiplied with a local signal LO bythe multiplier 102 to generate a multiplied signal. The low pass filter103 filters the multiplied signal to generate a baseband signal to beinput to the variable gain amplifier 104. In the variable gain amplifier104, the baseband signal is input to the amplifier 106 via the adder 105and amplified by the amplifier 106. The amplified signal is output as anoutput signal via an output terminal and fed back to the adder 105 viathe filter 107. In the above operation, when the gain of the amplifier106 is increased or decreased according to the signal from the filter103 by the gain controller 108, the cutoff frequency of the low passfilter 107 is decreased or increased by the gain controller 108simultaneously. As a result, the fluctuation of the lower cutofffrequency of the output signal is suppressed to realize a good offsetcanceling.

The present invention is not limited to the above embodiments and may bemodified appropriately. In the above embodiments, the gain F of thesub-amplifier circuit is constant. However, the gain F of thesub-amplifier circuit is not limited to be constant. In other words,when the gain of the main amplifier circuit 11 is A, the low limitcutoff frequency is AF/(2πCR). The purpose of the present embodiment isdirected to making this lower cutoff frequency constant if the gain A ischanged. Therefore, the gain F may be varied in the scope that does notdeviate from this purpose.

The variable gain amplifier related to the present embodiment is usedfor a mobile communication system that fluctuation of the lower signalbandwidth is not allowed in controlling a gain.

According to the present invention, there is provided a variable gainamplifier which suppresses fluctuation of the lower cutoff frequency dueto the change of a gain if the gain of a main amplifier is changed,realizes a good offset cancel, and can integrated in a semiconductorchip.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A variable gain amplifying method comprising: supplying to a variablegain amplifier an input signal and a feedback signal from a feedbackcircuit to amplify a difference between the input signal and thefeedback signal; and decreasing a cutoff frequency of the feedbackcircuit with an increase in a gain of the variable gain amplifiercircuit or vice versa.
 2. The method according to claim 1, whichincludes supplying, as the feedback signal, only a predetermined lowfrequency component in frequency components of an output signal of thevariable gain amplifier circuit.
 3. The method according to claim 1,which includes increasing the gain of the variable gain amplifiercircuit and decreasing the cutoff frequency of the feedback circuitsimultaneously or decreasing the gain of the variable gain amplifiercircuit and increasing the cutoff frequency of the feedback circuitssimultaneously.
 4. The method according to claim 1, wherein the feedbackcircuit comprises an operational amplifier, a variable capacitor and afirst resistor which are connected in parallel between an invertinginput terminal and output terminal of the operational amplifier, and asecond resistor connected between the inverting input terminal of theoperational amplifier and an output terminal of the variable gainamplifier circuit, a capacitance of the variable capacitor increasingwith an increase in the gain of the variable gain amplifier anddecreasing with a decrease of the gain of the variable gain amplifier.5. The method according to claim 1, wherein the feedback circuitcomprises an operational amplifier, a capacitor and a first resistorwhose resistance is variable and which are connected in parallel betweenan inverting input terminal of the operational amplifier and an outputterminal thereof, and a second resistor connected between the invertinginput terminal of the operational amplifier and an output terminal ofthe variable gain amplifier circuit, the resistance of the firstresistor increasing with an increase in the gain of the variable gainamplifier and decreasing with a decrease in the gain thereof.
 6. Themethod according to claim 5, wherein the second resistor comprises aresistor whose resistance is variable, the resistance of the firstresistor and the resistance of the second resistor increasing with anincrease in the gain of the variable gain amplifier and decreasing witha decrease in the gain thereof as a constant resistance ratio is keptbetween the first resistor and the second resistor.
 7. The methodaccording to claim 1, wherein the feedback circuit comprises a firstvoltage-current converter, a capacitor connected to an output terminalof the first voltage-current converter, and a second voltage-currentconverter whose mutual conductance is variable and which includes aninput terminal connected to the capacitor and an output terminal, theinput terminal and the output terminal being short-circuited, and themutual conductance of the second voltage-current converter decreasingwith an increase in the gain of the variable gain amplifier circuit orvice versa.
 8. The method according to claim 7, wherein a mutualconductance of the first voltage-current converter is variable, themutual conductance of the first voltage-current converter and the mutualconductance of the second voltage-current converter decrease at the samerate with an increase in the gain of the variable gain amplifier circuitor vice versa.
 9. The method according to claims 1, wherein the variablegain amplifier comprises a variable gain amplifier circuit to amplify adifference between the input signal and the feedback signal, a feedbackcircuit to supply the feedback signal to the variable gain amplifiercircuit, and a controller to control a cutoff frequency of the feedbackcircuit.
 10. A variable gain amplifying method comprising: supplying toa variable gain amplifier circuit an input signal and a feedback signalfrom a feedback circuit to amplify a difference between the input signaland the feedback signal; and controlling the gain of the variable gainamplifier circuit and a cutoff frequency of the feedback circuit to makea lower limit frequency of the output signal substantially constantregardless of variation of a gain of the variable gain amplifiercircuit.
 11. The method according to claim 10, which includesgenerating, as the feedback signal from the feedback circuit, only apredetermined low frequency component in frequency components of anoutput signal of the variable gain amplifier circuit.
 12. A variablegain amplifying method comprising: supplying to a variable gainamplifier circuit an input signal and a feedback signal from a feedbackcircuit to amplify a difference between the input signal and thefeedback signal, a gain of the variable gain amplifier circuit beingvaried according to a level of at least one of the output signal and theinput signal; and varying a cutoff frequency of the feed back circuitaccording to a variation of the gain of the variable gain amplifiercircuit to make a lower limit frequency of the output signalsubstantially constant.
 13. The method according to claim 12, whichincludes generating, as the feedback signal from the feedback circuit,only a predetermined low frequency component in frequency components ofan output signal of the variable gain amplifier circuit.